Microorganisms produce a variety of compounds which demonstrate anti-bacterial properties. One group of these compounds, bacteriocins, consists of bactericidal proteins with a mechanism of action similar to ionophore antibiotics. Bacteriocins are often active against species which are closely related to the producer. Their widespread occurrence in bacterial species isolated from complex microbial communities such as the intestinal tract, the oral or other epithelial surfaces, suggests that bacteriocins may have a regulatory role in terms of population dynamics within bacterial ecosystems. Bacteriocins are defined as compounds produced by bacteria that have a biologically active protein moiety and bactericidal action (Tagg et al., Bacteriological Reviews, Volume 40, 722 256, 1976). Other characteristics may include: (1) a narrow inhibitory spectrum of activity centered about closely related species; (2) attachment to specific cell receptors; and (3) plasmid-borne genetic determinants of bacteriocin production and of host cell bacteriocin immunity. Incompletely defined antagonistic substances have been termed “bacteriocin-like substances”. Some bacteriocins effective against Gram-positive bacteria, in contrast to Gram-negative bacteria, have wider spectrum of activity. It has been suggested that the term bacteriocin, when used to describe inhibitory agents produced by Gram-positive bacteria, should meet the minimum criteria of (1) being a peptide and (2) possessing bactericidal activity (Tagg et al., supra).
Lactic acid bacteria are among the most important probiotic microorganisms. They are Gram-positive, nonsporing, catalase-negative organisms devoid of cytochromes. They are anaerobic but are aerotolerant, fastidious, acid-tolerant, and strictly fermentative with lactic acid as the major end-product of sugar fermentation. Lactic acid producing bacteria include Lactobacillus species, Bifidobacterium species, Enterococcus faecalis, Enterococcus faecium, Lactococcus lactis, Leuconostoc mesenteroides, Pediococcus acidilactici, Sporolactobacillus inulinus, Streptococcus thermophilus, etc. These species are of particular interest in terms of widespread occurrence of bacteriocins within the group and are also in wide use throughout the fermented dairy, food and meat processing industries. Their role in the preservation and flavour characteristics of foods has been well documented. Most of the bacteriocins produced by this group are active only against other lactic acid bacteria, but several display anti-bacterial activity towards more phylogenetically distant Gram-positive bacteria and, under certain conditions, Gram-negative bacteria.
Lactobacilli have been extensively studied for production of antagonists. These include the well characterized bacteriocins (DeKlerk, Nature, Volume 214, 609, 1967; Upreti and Hinsdill, Antimicrob, Agents Chemother., Volume 7, 139 145, 1975; Barefoot and Klaenhammer, Antimicrob, Agents Chemother., Volume 45, 1808 1815 1983; Joerger and Klaenhammer, Journal of Bacteriology, Volume 167, 439 446, 1986, potential bacteriocin-like substances (Vincent et al., Journal of Bacterioll., Volume 78, 479, 1959), and other antagonists not necessarily related to bacteriocins (Valkil and Shahani, Bacteriology, Proc, 9, 1965; Hamdan and Milcolajeik, Journal of Antibiotics, Volume 8, 631636, 1974; Mikolajeik and Hamdan, Cultured Dairy Freducts, Page 10, 1975; and Shahani et al., Cultured Dairy Products Journal, Volume 11, 14 17, 1976).
Klaenhammer (FEMS, Microbiol, Rev., Volume 12, 39 86, 1993) has classified the lactic acid bacteria bacteriocins known to date into four major groups:
Group I: Lantibiotics which are small peptides of <5 kDa containing the unusual amino acids lanthionine and .beta.-methyl lanthionine. These are of particular interest in that they have very broad spectra of activity relative to other bacteriocins. Examples include Nisin, Nisin Z, carnocin U 149, lacticin 81, and lactocin 5. Group II-Small non-lanthionine containing peptides: a heterogeneous group of small peptides of <10 kDa. This group includes peptides active against Listeria spp. Group III-Large heat labile proteins of <30 kDa. Group IV-Complex bacteriocins-proteins containing additional moieties such as lipids and carbohydrates.
The lantibiotic peptide, nisin, is produced by Lactococcus lactis. Nisin is desired because it is a biodegradable antibacterial agent and is safe to use as a food preservative in processed dairy products. However, since nisin cannot be synthesized artificially, the only route of production is through fermentation and involves post-translational modifications.
With zero tolerance of bacterial contamination in food and feed processing, plus generally restrictive antibiotic usage, there is a need to develop a bacteriocin that can be utilized in food, feed, and medicine. Ideally, there is a need to develop a bacteriocin that has a wide range of antibacterial activity, especially against bacteria that are antibiotic resistant.
A disadvantage of conventional antibiotics is that they do not degraded easily. The remaining residue accumulates in the environment and promotes the emergence of multi-drug resistant bacterial strains. Naturally isolated polypeptides with bactericidal activities can be used as safe alternatives to synthetic antibiotics and are promising agents to effectively control bacterial infections without the disadvantage of antibiotics.